The purpose of the middle ear is to transmit pressure waves from the environment into the fluid-filled cochlea. While non-mammalian middle ears typically achieve this using a single relatively simple piston-like bone, the mammalian middle ear is unique in that it contains three distinct bones (ossicles)-the malleus, incus, and stapes-suspended within the middle-ear cavity by ligaments and tendons. In humans, among other mammals, these ossicles are connected via two synovial joints, and one muscle each is attached to the malleus and the stapes. Since this middle-ear design introduces additional flexibility beyond simple flexion of the ossicles, it might be expected to transfer vibrations less efficiently than a fused system. Why, then, have humans evolved this complicated and flexible middle-ear structure? An in-depth understanding of middle-ear flexibility will both further scientific knowledge and provide a basis for quantifying the effects of middle-ear pathologies. It will also lay a framework for the design of improved middle-ear prostheses, especially since many of the current columella-like devices remove much of the flexibility present in the healthy human middle ear. The central hypothesis proposed here is that the flexibility imparted by mobile joints in the human middle ear protects cochlear structures by reducing the peak input to the cochlea from impulsive stimuli and by allowing the two middle-ear muscles to reduce transmission to the cochlea during periods of sustained noise, and that this flexibility advantage does not come at the cost of an excessive adverse effect on normal sound transmission through the middle ear to the cochlea. This central hypothesis will be tested by 1) comparing 3D motion measurements of the normal human ossicular chain against measurements of modified versions with reduced flexibility due to fusing one or both ossicular joints or due to implanting a commercially available columella-type prosthesis, as well as through 2) the use of new, experimentally validated computational model of the human middle-ear joints and muscles. When completed, this proposal will contribute to our scientific understanding of the protective role of flexibility in the human ear, and will provide a new computational model of the middle ear. In addition to its usefulness for studying middle-ear flexibility, this model will also benefit futre research efforts, such as by providing a test bed for the design of more-effective middle-ear prostheses and surgical interventions.